Miniaturized tunable resonator comprising intermeshing concentric tubular members



United States Patent US. Cl. 333-82 4 Claims ABSTRACT OF THE DISCLOSUREA miniaturized tunable coaxial resonator for use at high frequencieswith high Q comprising intermeshing concentric tubular members in ashielded housing forming a folded coaxial line with capacitive loadingand a variable impedance coupling loop.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalities thereon or therefor.

Background of invention The present invention relates to resonantelectronic circuits and more particularly to coaxial resonant circuitsfor use with high frequencies.

At frequencies below approximately 100 megacycles, tuned circuitsemploying lumped inductance and capacitance are almost exclusively usedas resonators. On the other hand, at very high frequencies, i.e., inexcess of approximately 1000 megacycles, wave guides and cavities areemployed as resonators since they exhibit low losses, high Qs (ratio ofreactance to resistance) and their physical dimensions are acceptable.In this intermediate region of approximately 100 megacycles and 1000megacycles, various combinations of line sections and lumped reactancesare employed to provide acceptable size resonators with good Qs. Onesuch device called the helical resonator utilizes a section of coaxialcable with a coiled center conductor. Although this device reduces theoverall length of the cable required for resonance at a given frequency,as a result of the smaller diameter and greater length of the centerconductor, the losses associated with this device are quite high.Accordingly, such devices have only enjoyed limited success. A greatneed, therefore, exists for a high Q, low loss miniature resonator whichis completely shielded from external circuitry and has a high efiiciencyof operation in this region.

Summary of the invention It is therefore an object of the presentinvention to provide a low loss, high Q resonator of extrememiniaturization with high efficiency at selectably variable frequencies.These and other objects are achieved by providing a device havingcharacteristics of both the radial and axial coaxial lines. A radialline is defined herein as one in which the ratio of the diameter of theouter conductor is greater than the length and conversely for the axialline. To achieve these characteristics, the present invention employs ahousing assembly forming a completely shielded enclosure Within whichare fixed and movable concentric tubular members in intermeshingrelationship forming a serpentine-shaped radially extending chamberwhich simulates a folded coaxial line resonator by which an appreciablereduction in size is achieved. Relative movement between the tubularmembers varies the fre quency of operation thereby providing a readilytunable device. By the use of capacitive loading between the base of themovable tubular member and the interior of the housing, the size of theresonator is reduced still further. Input-output coupling to theresonator is provided by coupling loops or caps.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawing.

Brief description of the drawing FIG. 1 is a sectional elevation viewtaken along a diameter of a tunable resonator of the present invention;and

FIG. 2 is a sectional bottom view of the resonator taken on the line 2-2of FIG. 1 looking in the direction of the arrows.

Description of the preferred embodiment Referring to FIG. 1 of thedrawing, there is shown a coaxial resonator 10 of the present inventioncomprising a cylindrical housing member 11 of thin electricallyconductive material having a side wall 11a, a top end cover 11b whichmay be integrally formed with the housing member and a bottom end coverproviding a sealed cylindrical enclosure. A cylindrical member 12aaflixed at one end to the interior surface of the bottom end cover 11c,extends coaxially with the wall 11a of the housing, for about half thedistance thereof. As described hereinafter, more coaxially arrangedcylinders could be employed if desired. A movable assembly 13 having abase portion 1311 with cylinders 13b and 13c aflixed thereto, is inintermeshing concentric relation with the fixed cylindrical member 12a.As illustrated, it is preferable that the movable assembly 13 have onemore cylindrical memher than is fixed to the bottom end cover 11c sothat continuity of the coaxial line is maintained. Although the housingmember 11, the cylindrical member 12a and the cylinders 13b and areillustrated as having a circular cross section, it is to be understoodthat other tubular shapes such as rectangular, square, oval or othercombinations thereof could likewise be used without departing from thespirit of the present invention. It is therefore to be understood thatthe embodiment described herein is merely illustrative and is not to beconstrued by way of limitation.

Referring now to FIG. 2, the outer cylinder 13b is illustrated as havinga radial dimension greater than that of the cylinder 12a and thecylinder 13c is illustrated as having a radial dimension less than thecylinder 12a. The intermeshing concentric cylinders form aserpentineshaped chamber within the housing 11 which simulates afoldedcoaxial line, the length of which may be varied by varying thenumber of concentric cylinders, or by the dimensions of the housing. Theaverage length of this serpentine path is one-half the sum of thedistances between ABCDEFGH and AJKLMN and to a first approximation isbetween a and 71 wavelength because of the capacitive loading affordedby the capacitance between the top cover 11b and the base 13a, as willbe described hereinafter.

For purposes of illustration, the required length of a quarter-waveresonator at one hundred megacycles would be three-quarters of a meter.This length may be achieved from a fixed volume housing by increasingthe number of concentric intermeshing cylinders until the length of theserpentine path is equal to three-quarters of a meter. Using theapproximation described above, the capacitive loading, provided by thecapacitance between the base 13a and the top cover 11b, will reduce thislength to between and of a meter. By this arrangement, the ratio ofwavelength per unit volume is greatly increased over existingresonators. Whereas the length of the serpentine path afiects theresonant frequency of operation, the height or length of the housingwall 11a has little or no efiect on the resonant frequency so long asthe length is less than a quarter-wave length at the frequency ofoperation.

The length of a serpentine path is adjustable within the confines of thehousing by providing an adjustment post 13d having one end fixedcentrally to the base 130 and the other end threaded for engagement witha hearing 14 which is electrically and mechanically secured centrally tothe bottom cover 110. The bearing 14 is threaded to receive the threadedend of the adjustment post 13d so that a rotational movement of theadjustment post 13d, with the aid of an adjustment slot 13e, causes atranslational movement of the assembly 13 relative to the bottom cover110. This relative movement provides a fine adjustment of the resonantline length to enable operation of the device at a specific frequency.

Electrical connection between the housing member 11 and the movableassembly 13 is provided by a cylindrical sleeve 15 having leaf springfinger contactors 15a. The sleeve 15 is electrically and mechanicallysecured to the bottom cover 110 around the periphery of the bearing 14by solder or another appropriate bonding agent. The contactors 15aprovide sliding electrical contact between the adjustment post 13d andthe bottom cover 110, thereby short circuiting one end of the coaxialresonator. The exact point of contact between the finger contactors 15aand the adjustment post 13d is not critical except that since highelectrical currents exist at a short circuited end of a quarter-wavelength line (around the bearing 14) and a low resistance threadedcontact (between the adjustment post 13d and the bearing 14) is notreadily achievable, it is desirable to move the point of electricalcontact from this high current point to a lower current point.Accordingly, the exact point of contact of the finger contactors 15a ismerely one of design choice.

Electrical coupling to the coaxial resonator is provided by a couplingloop 16 affixed at one end to a point b on the sleeve 15, and at theother end to an electrical connector 17 secured to the bottom cover 110.The impedance which the resonator exhibits to an input signal is afunction of the position of the electrical contactor 15b along thesleeve 15; that as, as the point of connection is moved away from thebottom cover 110 toward the finger contactor 1511, the impedanceincreases. Accordingly, the impedance which the resonator displays to anexternal circuit connected to the connector 17 can be changed merely bychanging the point of connection 1517. It is to be understood that morethan one loop could be employed if so desired or that other couplingtechniques, such as rotatable coupling loops, could be equally wellemployed without deviating from the spirit of the present invention.

To further reduce the physical size of the resonator and hence the ratioof length of the serpentine path to the wavelength, reactive loading isprovided. In the embodiment described in FIG. 1, this loading isaccomplished by the relatively large capacitance existing between thebase 13a and the top cover 11b of the cylindrical housing 11. As themovable assembly 13 approaches the top cover 11b, capacitive loadingincreases inversely with the distance between them. A dielectricmaterial 18 is secured to the inner surface of the top cover 11b so thatmaximum capacitance can be achieved without fear of short circuiting theresonator; however, the presence of the dielectric material is notessential to the operation of the resonator and may be omitted ifdesired.

In summary, the resonator of the present invention provides a high Qminiaturized tunable resonator for use at frequencies betweenapproximately 100 and 1000 megacycles. By increasing the number ofconcentric tubular members, the wavelength per unit volume is increasedconsiderably over previously described devices. Additionally, byutilizing the capacitance between the top end cover 11b and the base13a, maximum loading per unit area is provided in the same device.Further, the enclosed assembly acts as a shield for the resonator sothat coupling between the resonator and adjacent circuitry is minimizedwithout the need for additional shielding.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

I claim:

1. An apparatus for resonating at high frequencies comprising:

an electrically conductive tubular housing having first and secondelectrically conductive end covers attached thereto;

a trough-shaped tubular member having a base with side walls securedthereto;

a first tubular member having a radial dimension less than that of saidside walls and concentric therewith, said first tubular member afiixedalong an end to said base;

a second tubular member aflixed to said first end cover andconcentrically intermeshing with said first tubular member and the sidewalls of said trough-shaped member, said intermeshing members therebyforming a resonant serpentine chamber;

an adjustment post secured at one end to said base and movable from theother end relative to said first end cover, whereby an axial movement ofsaid adjustment post varies the resonant frequency of said serpentinechamber; and

means to couple signals to said housing including a sleeve concentricwith said adjustment post aflixed at one end to said first end cover andhaving finger contactors at the other end thereof in electrical contactwith said post whereby a sliding electrical contact between said sleeveand said post is provided.

2. An apparatus as recited in claim 1 wherein said means to couplesignals to said housing further comprises:

a coupling loop connected at one end to a point on said sleeve; and

an electrical connector mounted in said first end cover and electricallyconnected to the other end of said coupling loop for providingelectrical access to said resonant serpentine chamber.

3. An apparatus for resonating at high frequencies as recited in claim 2wherein:

said base forms a first plate of a capacitor; and

said second end cover forms a second plate of said capacitor, thecapacitance between said base and said second end cover increasing theresonant wavelength of said chambered housing thereby reducing the ratioof the length of the serpentine chamber to the wavelength.

4. An apparatus as recited in claim 3 further comprising:

means to provide a dielectric material between said second end cover andthe base of said trough-shaped member.

References Cited UNITED STATES PATENTS 2,111,219 8/1939 Malter.

2,181,901 12/ 1939 Lindenblad.

2,363,641 11/ 1944 Carlson.

2,500,875 3/ 1950 Schupbach.

2,603,754 7/ 1952 Hansen.

HERMAN KARL SAALBACK, Primary Examiner.

PAUL L. GENSLER, Assistant Examiner.

US. Cl. X.R.

